Hybrid electric vehicles (HEVS) combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle. This results in an increase in fuel economy over conventional vehicles. This combination also offers extended range and rapid refueling that users expect from a conventional vehicle, with a significant portion of the energy and environment benefits of an electric vehicle. The practical benefits of HEVs include improved fuel economy and lower emissions compared to conventional vehicles. The inherent flexibility of HEVs also permits their use in a wide range of applications, from personal transportation to commercial hauling.
A parallel hybrid electric vehicle requires that a power path for both constant and variable power be present. That is, a parallel hybrid electric vehicle uses power from both a mechanical source such as an internal combustion engine as well as an electrical source. This permits the HEV to use a smaller engine as the mechanical source. The smaller engine size and system operating characteristics provide even greater performance or improved fuel economy with lower emission. A significant challenge, however, in the design of HEVs, has been to produce a drive system that takes advantage of the high efficiency of mechanical components and the versatility of electrical components.
In the past, various types of parallel hybrid systems have been proposed for multiple use applications such as automobiles. For example, planetary gear sets have been used in automatic transmissions for many years. However, most automatic transmissions use a double planetary gear set such as a Simpson or Ravigneaux set. The typical automatic transmission uses only a single power source for the vehicle. Accordingly, it is desirable to provide a drive system which allows the system to operate at its most efficient power transmission point where the system spends most of its time while providing a means of generating the torque required to accelerate the vehicle without having a multi-gear ratio transmission. It is further desirable to provide a drive system that enables each source (mechanical or electrical) in the system to operate either independently or in conjunction with one another for transferring power to an output device.